Mandrel mill rolling is a step in the process for producing seamless pipes or tubes by hot working. A hollow shell is rolled by being axially transferred through multiple stands of grooved-rolls while supported on a mandrel bar. The surface of the mandrel bar is usually coated with lubricant that is solid lubricant. Satisfactory lubrication is difficult because the mandrel bar undergoes harsh sliding friction against the inner surface of the shell. This type of rolling damages the surface of the mandrel bar resulting in repeated friction, seizure, roughening, or cracking that shortens the mandrel bar service life.
The mandrel bar is made of hot working tool steel such as SKD6 and SKD61. The steel used for a mandrel bar is polished on its surface and then quenched and tempered in order to adjust the hardness and develop a scale on the surface.
Recent rising demand for high-alloy steel pipes such as of 13% chromium steel has revealed some problems, one of which is seizure on the mandrel bar. That is because high alloy steel has high deformation resistance and it is difficult to form oxidized scale on the inner surface of the hollow shell made of the high alloy steel. Such problems are handled by applying lubricant to the surface of the mandrel bar during rolling of the high alloy steel, though seizure cannot be avoided altogether. The seizure might cause oscillations during rolling, exerting adverse effects on finished dimensions and causing mandrel bar stripping defects. This seizure results in axial streaking flaws occurring on the inner surface of the hollow shell that require a large amount of time and money to repair.
Patent document 1 proposes a mandrel bar for manufacturing hot-worked seamless tube having a nitride treated layer with an average roughness of 0.5 to 5.0 μm along the axial center line of the surface of its base material in order to resolve these problems.
Patent document 2 proposes a method for producing a piercing mandrel bar for rolling seamless stainless tube characterized by the following steps;
diffusing Cu, Zn, Al, or an alloy thereof over the frictional surface of a mandrel bar formed in a piercing shape for use in rolling seamless steel tube, thereby embrittling the grain boundaries on the frictional surface and subjecting heat treatment for thermal refining; or
generating fine hexagonal surface cracking on the embrittled grain boundaries by rapid heating at high temperature in the initial rolling stage.    [Patent document I] Japanese Unexamined Patent Application Publication No. 6-262220    [Patent document 2] Japanese Unexamined Patent Application Publication No. 6-339709